Recently, an image-recording material has been dominated particularly by a material for forming a color image and specifically, a recording material of inkjet system, a recording material of heat-sensitive transfer system, a recording material of electrophotographic system, a silver halide photosensitive material of transfer system, a printing ink, a recording pen and the like are popularly used. Further, in an image device such as CCD of photographing equipment or in a display such as LCD and PDP, a color filter is used for recording and reproducing a color image. In these color image recording materials and color filters, colorants (dyes or pigments) using three primary colors for a so-called additive or subtractive color mixing method are used for displaying or recording a full color image, but a colorant having absorption characteristics capable of realizing a preferred color reproduction region and having fastness enough to endure various use conditions or environmental conditions has not been found until now, and accordingly, improvements are keenly demanded.
The dyes and pigments used in each of the aforementioned uses need to commonly have the following properties. That is, the dyes and pigments need to have preferred absorption characteristics for color reproduction, and exhibit good fastness under environmental conditions used, for example, light resistance, heat resistance, resistance to oxidative gases such as ozone and the like. In addition, in the case where the colorant is a pigment, the colorant also need to have properties such as being substantially insoluble in water or an organic solvent, showing good chemical resistance, and not impairing the preferred absorption characteristics in the molecular dispersion state even when used as a particle. The requisite characteristics may be controlled by varying the degree of the intermolecular interaction, but both characteristics are in a trade-off relationship and therefore, it is difficult to satisfy both at the same time.
Furthermore, in using a pigment, other than the properties described above, the pigment also need to have a particle diameter and a particle shape necessary for bringing out a desired transparency, to show good fastness under environment conditions used, for example, light resistance, heat resistance, resistance to an oxidative gas such as ozone, water resistance, and chemical resistance to an organic solvent, a sulfurous acid gas or the like, and to have properties capable of being dispersed even into microparticles in a medium used and keeping the dispersed state stable.
That is, compared to the dye which is required to have performances as a colorant molecule, performances required for the pigment need to satisfy not only performances as a colorant molecule, but also the aforementioned requisite performances as a solid (fine particle dispersion) as an aggregate of colorant molecules in various fields. As a result, a compound group which may be used as a pigment is extremely limited as compared to the dye, and thus even when a high-performance dye is derived into a pigment, a pigment capable of satisfying the requisite performances as a fine particle dispersion is very few in number and may not be easily developed. This is also confirmed from the fact that the number of pigments registered in the Color Index is less than 1/10 of the number of dyes.
Among the pigments, particularly an azo pigment has high lightness and excellent light resistance and heat resistance, and thus is widely used as a pigment for a printing ink, an inkjet ink, an electrophotographic material, and a color filter. Further, as the use is expanded, the pigment also needs to require much better stability over time regardless of a medium used than stability at a level usually used in a printing ink, a gravure ink, and a coloring agent. For example, Patent Document 1 discloses an azo pigment with a specific structure including a pyrazole ring, in which the azo pigment has excellent color characteristics such as color and excellent light resistance.
Simultaneously, in the use of a color filter or an inkjet ink, it is required to further enhance clearness or transparency. In order to enhance clearness or transparency, it is effective to finely disperse a pigment, and therefore, a method of efficiently preparing a pigment fine particle by which a fine dispersion may be formed is also required.
As a method of preparing an organic pigment fine particle, for example, there is a method of obtaining fine and size-regulated particles by selecting an appropriate reaction conditions during synthesis like the azo pigment. In addition, there are a method of preparing a pigment from very fine and aggregated particles produced during synthesis by subjecting the particles to particle growth and size-regulating the particles in subsequent processes like a copper phthalocyanine green pigment, and a method of preparing a pigment from crude and uneven particles produced during synthesis by finely pulverizing and size-regulating the particles in subsequent processes like a copper phthalocyanine blue pigment. For example, a diketopyrrolopyrrole pigment is generally synthesized as a crude pigment by reacting succinic diester with aromatic nitrile in an organic solvent (see, for example, Patent Document 2). Then, the crude diketopyrrolopyrrole pigment is heat-treated in water or an organic solvent, followed by pulverization such as wet grinding into a form suitable for use (see, for example, Patent Document 3). Furthermore, Patent Document 4 discloses a method of preparing an azo pigment with a specific structure, which includes a pyrazole ring, which may be prepared at a high efficiency and a low cost.
Further, among organic pigments, there are some pigments showing polymorphism, and it is known that these pigments may take two or more crystal forms even though these pigments have the same chemical composition. For example, in the C.I. Pigment Red 254, an α-crystal form and a β-crystal form are known. In addition, in the C.I. Pigment Yellow 181 which is an azo pigment, various types of crystal forms are known (see, for example, Patent Document 6).